Re: [HACKERS] Block level parallel vacuum

Hi
I took all attached patches(v32-01 to v32-4) and one Dilip's patch from
"Questions/Observations related to Gist vacuum" mail thread. On the top of
all these patches, I created one more patch to test parallel vacuum
functionally for all existence test suite.
For reference, I am attaching patch.

*What does this patch?*
As we know that if we give parallel option with vacuum, then only we are
vacuuming using parallel workers. So to test, I used existence guc
*force_parallel_mode* and tested parallel vacuuming.

If force_parallel_mode is set as *regress, *then if parallel option is not
given with vacuum, I am forcing to use parallel workers for vacuum. If
there is only one index and parallel degree is not given with vacuum(or
parallel option is not given), and *force_parallel_mode = regress*, then I
am launching one parallel worker(I am not doing work by leader in this
case), but if there is more than one index, then i am using leader as a
worker for one index and launching workers for all other indexes.

After applying this patch and setting *force_parallel_mode = regress,* all
test cases are passing (make-check world)

I have some questions regarding my patch. Should we do vacuuming using
parallel workers even if force_parallel_mode is set as *on, *or we should
use new GUC to test parallel worker vacuum for existence test suite?

Please let me know your thoughts for this patch.

Thanks and Regards
Mahendra Thalor
EnterpriseDB: http://www.enterprisedb.com

On Tue, 29 Oct 2019 at 12:37, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:

> On Mon, Oct 28, 2019 at 2:13 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Thu, Oct 24, 2019 at 4:33 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com>
> wrote:
> > >
> > > On Thu, Oct 24, 2019 at 4:21 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> > > >
> > > > On Thu, Oct 24, 2019 at 11:51 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com>
> wrote:
> > > > >
> > > > > On Fri, Oct 18, 2019 at 12:18 PM Dilip Kumar <
> dilipbalaut(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > On Fri, Oct 18, 2019 at 11:25 AM Amit Kapila <
> amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > > > > >
> > > > > > > I am thinking if we can write the patch for both the
> approaches (a.
> > > > > > > compute shared costs and try to delay based on that, b. try to
> divide
> > > > > > > the I/O cost among workers as described in the email above[1])
> and do
> > > > > > > some tests to see the behavior of throttling, that might help
> us in
> > > > > > > deciding what is the best strategy to solve this problem, if
> any.
> > > > > > > What do you think?
> > > > > >
> > > > > > I agree with this idea. I can come up with a POC patch for
> approach
> > > > > > (b). Meanwhile, if someone is interested to quickly hack with
> the
> > > > > > approach (a) then we can do some testing and compare.
> Sawada-san,
> > > > > > by any chance will you be interested to write POC with approach
> (a)?
> > > > > > Otherwise, I will try to write it after finishing the first one
> > > > > > (approach b).
> > > > > >
> > > > > I have come up with the POC for approach (a).
> >
> > > > Can we compute the overall throttling (sleep time) in the operation
> > > > separately for heap and index, then divide the index's sleep_time
> with
> > > > a number of workers and add it to heap's sleep time? Then, it will
> be
> > > > a bit easier to compare the data between parallel and non-parallel
> > > > case.
> > I have come up with a patch to compute the total delay during the
> > vacuum. So the idea of computing the total cost delay is
> >
> > Total cost delay = Total dealy of heap scan + Total dealy of
> > index/worker; Patch is attached for the same.
> >
> > I have prepared this patch on the latest patch of the parallel
> > vacuum[1]. I have also rebased the patch for the approach [b] for
> > dividing the vacuum cost limit and done some testing for computing the
> > I/O throttling. Attached patches 0001-POC-compute-total-cost-delay
> > and 0002-POC-divide-vacuum-cost-limit can be applied on top of
> > v31-0005-Add-paralell-P-option-to-vacuumdb-command.patch. I haven't
> > rebased on top of v31-0006, because v31-0006 is implementing the I/O
> > throttling with one approach and 0002-POC-divide-vacuum-cost-limit is
> > doing the same with another approach. But,
> > 0001-POC-compute-total-cost-delay can be applied on top of v31-0006 as
> > well (just 1-2 lines conflict).
> >
> > Testing: I have performed 2 tests, one with the same size indexes and
> > second with the different size indexes and measured total I/O delay
> > with the attached patch.
> >
> > Setup:
> > VacuumCostDelay=10ms
> > VacuumCostLimit=2000
> >
> > Test1 (Same size index):
> > create table test(a int, b varchar, c varchar);
> > create index idx1 on test(a);
> > create index idx2 on test(b);
> > create index idx3 on test(c);
> > insert into test select i, repeat('a',30)||i, repeat('a',20)||i from
> > generate_series(1,500000) as i;
> > delete from test where a < 200000;
> >
> > Vacuum (Head) Parallel Vacuum
> > Vacuum Cost Divide Patch
> > Total Delay 1784 (ms) 1398(ms)
> > 1938(ms)
> >
> >
> > Test2 (Variable size dead tuple in index)
> > create table test(a int, b varchar, c varchar);
> > create index idx1 on test(a);
> > create index idx2 on test(b) where a > 100000;
> > create index idx3 on test(c) where a > 150000;
> >
> > insert into test select i, repeat('a',30)||i, repeat('a',20)||i from
> > generate_series(1,500000) as i;
> > delete from test where a < 200000;
> >
> > Vacuum (Head) Parallel Vacuum
> > Vacuum Cost Divide Patch
> > Total Delay 1438 (ms) 1029(ms)
> > 1529(ms)
> >
> >
> > Conclusion:
> > 1. The tests prove that the total I/O delay is significantly less with
> > the parallel vacuum.
> > 2. With the vacuum cost divide the problem is solved but the delay bit
> > more compared to the non-parallel version. The reason could be the
> > problem discussed at[2], but it needs further investigation.
> >
> > Next, I will test with the v31-0006 (shared vacuum cost) patch. I
> > will also try to test different types of indexes.
> >
>
> Thank you for testing!
>
> I realized that v31-0006 patch doesn't work fine so I've attached the
> updated version patch that also incorporated some comments I got so
> far. Sorry for the inconvenience. I'll apply your 0001 patch and also
> test the total delay time.
>
> Regards,
>
> --
> Masahiko Sawada
>